Graphite interlaminar compound and method for producing the same

ABSTRACT

Graphite intercalation compound interposing at least PbCl 2  interlaminarly among graphite, synthesized by mixing a raw material graphite, PbCl 2 , and a metal halide other than PbCl 2  and heating the mixture.

This application is a continuation of application Ser. No. 07/785,853,filed Oct. 31, 1991, now abandoned.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a graphite intercalation compoundhaving a lower resistivity for use as a conductive material, morespecifically to a graphite intercalation compound containing a metalhalide as the guest and also relates to a method for producing the same.

2) Description of the Related Art

Graphite intercalation compounds are compounds wherein atoms, moleculesand ions are intercalated into graphite. The practical value thereof hasbeen drawing attention recently because of the improved functionalproperties such as the low resistivity comparable to metal and theexcellent discharge and frictional properties and also because of therelatively easy synthesis.

Because the graphite intercalation compounds, incorporating as theintercalates metal halides such as FeCl₃, CuCl₂ and the like,demonstrate a higher conductivity and are substantially stable in theatmosphere, the application thereof as conductive materials is nowexpected.

It is known that the graphite intercalation compounds containing a metalhalide as the intercalate can be synthesized and produced by reactinggraphite with a metal halide at a predetermined temperature according toa mixing method.

The present inventors have carried out the environmental test of knowngraphite intercalation compounds (abbreviated as GIC often hereinafter)containing metal halides as the intercalate in an atmosphere of a highhumidity. They have found that in such an atmosphere the change withtime of the resistance value of NiCl₂ --CuCl₂ --GIC, otherwiseconsidered to be stable, is large.

In order to apply a conductive paste containing a metal halide GICdispersed in an organic binder to an electronic device, it must beguaranteed that the properties thereof won't change in actualenvironment for a long period of time. It is certainly known that theGIC is stable to heat up to about 200° C. However, when an environmentaltest was carried out in such a manner that the paste was printed andthermally cured to experimentally make an electric conductor which wasthen left to stand in an atmosphere of a high humidity of 90% RH at 40°C., the known metal halide GIC, considered as a stable compound in theatmosphere, was found to have a non-negligibly larger change of theresistance value with time. In the case of an electric conductorexperimentally made from FeCl₃ --GIC, the resistance value thereofincreased by 30% when 100 hours passed.

Conventionally known metal halide GICs have inadequate stability in anatmosphere of high humidity as has been described above, which is aserious drawback for putting such GICs into practical use as conductivematerials.

SUMMARY OF THE INVENTION

It is a first objective of the present invention to provide a metalhalide graphite intercalation compound with great moisture-resistantstability. It is a second objective of the present invention to providea method for producing such a graphite intercalation compound.

The first objective of the present invention described above is achievedby intercalating a guest containing at least PbCl₂ into graphite havinga laminar crystalline structure.

There has been found no example in which PbCl₂ is intercalated intographite. It has been considered that a graphite intercalation compoundwon't be formed from PbCl₂ alone under any condition. According to thepresent invention, however, a mixture of PbCl₂ and a second metal halidesingly capable of reacting with graphite to form a graphiteintercalation compound, is reacted with graphite under predeterminedsynthesis conditions, whereby the second metal halide simultaneouslyincorporates PbCl₂ interlaminarly in the graphite during the reaction ofthe former with graphite, so that both the PbCl₂ and the metal halideother than PbCl₂ can be intercalated into graphite.

The second objective of the present invention described above can beachieved by mixing a raw material graphite with PbCl₂ and a metal halideother than PbCl₂, heating the mixture at a temperature between 300° C.and 550° C., thereby reacting the mixture together for a period of timesufficient to increase the weight of the raw-material graphite by thesynthesis of a graphite intercalation compound.

According to the experiments of the present inventors, it has beenconfirmed that a graphite intercalation compound, synthesized so thatthe compound contains at least PbCl₂ as the guest, not only demonstratesa higher conductivity but also a remarkably improved moisture-resistantstability compared with the stability of known metal halide GICs. Thisis due to the fact that most metal halides are deliquescent and easilysoluble in water, while PbCl is exceptionally not deliquescent and isslightly soluble in water. The graphite intercalation compound isconsidered to have excellent moisture-resistant stability, as describedin detail hereinafter, due to such properties of PbCl₂. That is, theinterculated substance (metal halide) is known to have nearly the samestructure as the one prior to the insertion, which is a possible reasonwhy the resulting graphite intercalation compound has excellentmoisture-resistant stability.

According to the results confirmed by the present inventors,furthermore, it is indicated that the moisture-resistant stability isrelated to the amount of PbCl₂ intercalated into the graphiteintercalation compound. PbCl₂ and FeCl₃ are employed as intercalates inone example as shown in FIG. 5. In FIG. 5, the increase in the molarratio of Pb/Fe in an intercalated substance causes the increase in themoisture-resistant stability. If the molar ratio of Pb/Fe is 0.05 ormore, the graphite intercalation compound has a highermoisture-resistant stability than conventional FeCl₃ --GIC. If the ratiois 0.24 or more, the compound demonstrates a moisture-resistantstability sufficient for practical use; and if the ratio is 0.5 or more,the moisture-resistant stability is improved up to a value identical tothat of the graphite host.

In accordance with the present invention, the metal halide concurrentlyused with PbCl₂ enables the intercalation of PbCl in graphite. Halidessuch as FeCl₃, CuCl₂, AlCl₃, GaCl₃, CoCl₂, MnCl₂, CrCl₃, MoCl₅, CdCl₂and the like, are used. Among them, FeCl₃ and CuCl₂ are preferable inthat no introduction of toxic chlorine gas from outside is requiredbecause they generate chlorine gas by disproportionation and in that themost appropriate temperature range for reacting graphite with them,namely 300° C. to 550° C., is industrially advantageous.

For the production of the graphite intercalation compound of the presentinvention, the ratio of host and guest, the charge ratio in the guest ofPbCl₂ and other metal halides, and the synthesis conditions areimportant. If the conditions vary, the properties of a graphiteintercalation compound thus obtained are different.

The ratio of PbCl₂ in the guest of a graphite intercalation compound,which controls the moisture-resistant stability, is related to both thecharge ratio of the materials PbCl₂ and another metal halide and to thesynthesis temperature. A larger charge ratio of PbCl₂ and a highersynthesis temperature cause an increase in the amount of Pb in agraphite intercalation compound, thus leading to the moisture-resistantstability. In the case where natural graphite of an average particlesize of 10 μm is employed as the host while PbCl₂ and FeCl₃ are employedas the guests and the synthesis is effected in a sealed tube, forexample, a small amount of PbCl₂ is intercalated into graphite alongwith FeCl₃ at a synthesis temperature of 300° C. or more, as shown inthe region formed by dashed lines in FIG. 6. In the region with slantinglines (Region A), there can be obtained a graphite intercalationcompound having moisture-resistant stability equal to the stability ofthe host graphite or having sufficient stability for practical use. Inthe region most preferable for practical use, the upper limit of thesynthesis temperature is 954° C., which is determined by the fact thatthe vaporization of the metal halide used as a reactive substanceremarkably increases the pressure inside a reaction vessel, causing adangerous situation and by the boiling point of PbCl₂ being 954° C.

The charge ratio of PbCl₂ and FeCl₃ is in the range from PbCl₂ of 95%and FeCl₃ of 5% to PbCl₂ of 40% and FeCl₃ of 60%. This range isdetermined because only an extremely small amount of FeCl₃ is reactedwith graphite due to the insufficiency of FeCl₃ which works to introducePbCl₂ interlaminarly among graphite in the case where the ratio of FeCl₃is too small in the mixture, and because FeCl₃ is preferentially reactedwith graphite so that almost no PbCl₂ is inserted among graphite in thecase where the ratio of FeCl₃ is too large.

In the case where a metal halide other than FeCl₃ is used together withPbCl₂, the graphite intercalation compound as the objective can beobtained by modifying the synthesis conditions appropriately.

BRIEF DESCRIPTION OF THE DRAWINGS

All the graphs relate to the embodiments of the present invention; FIG.1 is a characteristic graph of the moisture-resistant stability ofraw-material graphite; FIG. 2 is a characteristic graph representing themoisture-resistant stability of each sample synthesized at a temperatureof 300° C.; FIG. 3 is a characteristic graph representing themoisture-resistant stability of each sample synthesized at a temperatureof 450° C.; FIG. 4 is a characteristic graph representing themoisture-resistant stability of each sample synthesized at a temperatureof 540° C.; FIG. 5 is a graph representing the molar ratio of Pb/Fecontained in the GIC of each sample and the results of the evaluationthereof after 250 hours in humidity; and FIG. 6 is a graph wherein thesynthesis condition of each sample is plotted on the graph representingthe state of PbCl₂ - FeCl₃.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will now be explainedhereinafter.

Example 1

By employing natural graphite of an average particle size of 10 μm asthe host and modifying the charge ratio and the synthesis temperature,nine types of PbCl₂ --FeCl₃ --GIC were synthesized according to a mixingmethod. The raw material graphite, PbCl₂ and FeCl₃ were placed and mixedtogether at a predetermined charge ratio in a Pyrex-glass reaction tube.The tube and its contents were dehydrated by heating at 120° C. for onehour under vacuum suction. Then, the reaction tube was sealed by glassfusion and heated at a predetermined reaction temperature between 300°C. and 540° C. for 24 hours. The resulting reaction product wasrepeatedly washed in boiling water and methanol, to remove the unreactedmetal halide on the sample surface to obtain samples Nos. 1 to 9(graphite intercalation compounds) shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________        Charge ratio             Pb/Fe ratio                                                                          Weight of                                     (molar ratio)                                                                        Synthesis                                                                            Heating    after synthe-                                                                        guest to                                  Sample                                                                            graphite:                                                                            Temperature                                                                          time Resistivity                                                                         sis (molar                                                                           weight of                                 No. PbCl.sub.2 :FeCl.sub.3                                                               (°C.)                                                                         (H)  (mΩ · cm)                                                            ratio) host (%)                                  __________________________________________________________________________    1   5:0.8:1.2                                                                            300    24   4.0   0.0056 43.4                                      2   5:1:1  300    24   4.6   0.0089 68.2                                      3   5:1.2:0.8                                                                            300    24   4.0   0.0130 77.4                                      4   5:0.8:1.2                                                                            450    24   3.4   0.1221 96.0                                      5   5:1:1  450    24   3.5   0.1480 112.6                                     6   5:1.2:0.8                                                                            450    24   3.3   0.5690 83.2                                      7   5:0.8:1.2                                                                            540    24   3.2   0.0571 81.1                                      8   5:1:1  540    24   3.2   0.2424 102.2                                     9   5:1.2:0.8                                                                            540    24   3.8   0.6070 87.2                                      __________________________________________________________________________

The resistivity of each sample was measured according to four probemethod comprising molding under the state of pressure loaded to eachpowdery sample and measuring a compressed potential difference. Theresults are also shown in Table 1. The resistivity of a raw materialgraphite was 8.6 mΩ.cm.

As is shown in the Table, there is observed a tendency that the ratio ofPb to Fe in the synthesized graphite intercalation compound is higher ata larger charge ratio of PbCl₂ and a higher synthesis temperature,namely the tendency that PbCl₂ is more easily intercalated intographite.

As to the resistivity, there was observed a tendency that theresistivity got lower as the weight of guest to that of host wasincreased, and that the resistivity was about 1/2 to 1/3 that of thehost if the weight of the guest was 40% or more. Thus, the resultantgraphite intercalation compound showed a conductivity almost as high asthat of FeCl₃ --GIC. The values shown in Table 1 are the resistivitiesof the pressed powder compacts of the samples, including the contactresistance between particles, so the values are possibly slightly largerthan the inherent resistivities of the products.

Then, the samples No. 1 to 9 shown in Table 1 were individuallydispersed in an organic binder (phenolic resin) to be changed intopastes, which were then printed and thermally cured to experimentallymake electric conductors. These electric conductors were left to standin an atmosphere of a high humidity of 90% RH at 40° C., to measureresistance values with time for the evaluation of moisture-resistantstability of each sample. The results are shown in FIGS. 1 to 6. FIG. 1shows the results of the measurement of the raw-material graphite; FIG.2 shows the results of the measurement of samples Nos. 1 to 3; FIG. 3shows the results of the measurement of samples Nos. 4 to 6; FIG. 4shows the results of the measurement of samples Nos. 7 to 9; FIG. 5shows the results of the measurement of the molar ratio of Pb/Fe in theGICs of samples Nos. 1 to 9, along with the results of their evaluationafter 250 hours in humidity; FIG. 6 shows a graph where the synthesisconditions of samples Nos. 1 to 9, namely charge ratio and synthesistemperature, are plotted on the graph showing the state of PbCl₂--FeCl₃. In FIG. 6, the numerical figure above each point shows thesample number and the numerical figure under each point shows theresults of the evaluation of the percentage change of the resistancevalue after 250 hours in humidity. For comparison, a conductive materialwas experimentally made of the paste with FeCl₃ --GIC being dispersed inan organic binder, and the percentage change of the resistance value ofthe electric conductor was then measured under the same condition. Evenafter 100 hours, the percentage change of the resistance value was notsaturated, and the resistance value increased by 27% on average comparedwith the initial value.

As shown in FIGS. 2 to 6, the electric conductor incorporating PbCl₂--FeCl₃ --GIC as the conductive material had a lower rate of change ofthe resistance value with time in an atmosphere of high humidity, andthe GIC synthesized under the synthesis condition shown in Region A ofFIG. 6 contains Pb of 0.2 or more as a molar ratio of Pb and Fe in theGIC. The electric conductor shows excellent moisture-resistant stabilityalmost equal to the stability of the raw-material graphite or a degreeof stability sufficient for practical use.

There are explained hereinbelow examples of syntheses wherein the chargeratio of host and guest and the synthesis temperature, among thesynthesis conditions, are modified.

Example 2

One gram of natural graphite powder of an average particle size of 400μm, 3.38 g of FeCl₃ and 5.79 g of PbCl₂ were mixed together and reactedat 300° C. in a nitrogen stream of 20 ml/min for one hour. After washingin heated aqueous hydrochloric acid, the sample thus obtained wasanalyzed with energy dispersive spectroscopy. It was observed that bothof the chlorides PbCl₂ and FeCl₃ were intercalated into graphite andformed a stage 4 GIC even under this synthesis condition.

Example 3

One gram of natural graphite powder of an average particle size of 400μm, 3.38 g of FeCl₃ and 5.79 g of PbCl₂ were mixed together and reactedat 250° C. in a nitrogen stream of 15 ml/min for one hour. As in Example2, it was confirmed in this case that PbCl₂ along with FeCl₃ wasintercalated into graphite and formed a stage 2 GIC.

As has been described above, PbCl₂ and other metal halides are mixed ata predetermined ratio with graphite. The resulting mixture is reactedtogether at a predetermined temperature to synthesize a graphiteintercalation compound, to enable the PbCl₂ to be intercalated, so therecan be realized a higher stability in an atmosphere of high humidity,which is not provided by conventional metal halide GICs, in addition tothe high conductivity as the advantage of conventional metal halideGICs. Furthermore, the raw materials are not costly and the productionthereof is easy. The application thereof as an excellent conductivematerial can be expected. That is, there can be produced in inexpensivefashion a resistor with a lower resistance and a conductive materialwith high conductivity, which properties are not easily changed in anenvironment of higher temperatures and higher humidities, so thepractical value thereof is extremely high.

As has been explained insofar, a novel graphite intercalation compoundcan be produced by intercalating at least PbCl₂ into in graphite inaccordance with the present invention. The graphite intercalationcompound not only demonstrates a higher conductivity but also hasexcellent moisture-resistant stability, so the compound is highly usefulas a conductive material. Therefore, the present invention can provide agraphite intercalation compound having extremely high practical effects,and the method for producing the same.

What is claimed is:
 1. A graphite intercalation compound comprisingPbCl₂ and FeCl₃ intercalated into graphite, wherein the molar ratio ofPb to Fe is at least 0.5, and wherein the combined weight of the PbCl₂and FeCl₃ is at least 40% of the weight of the graphite.
 2. A method forproducing a graphite intercalation compound comprising:mixing a rawmaterial comprising graphite with PbCl₂ and FeCl₃ to or a mixture,wherein the molar percentage of PbCl₂ is between 40 and 95 percent ofthe sum of PbCl₂ and FeCl₃, and wherein the weight of the sum of thePbCl₂ and FeCl₃ is at least 40% of the weight of the graphite; andheating the mixture for a sufficient time at a temperature between 300°C. and 550° C. to intercalate PbCl₂ and FeCl₃ into graphite.
 3. Themethod of claim 2 in which the molar percentage of PbCl₂ and thetemperature are within Region A of FIG.
 6. 4. The graphite intercalationcompound of claim 1 wherein the molar ratio is at least 0.24.
 5. Thegraphite intercalation compound of claim 1 wherein the molar ratio is atleast 0.5.